As far as I can tell, the standard material for Cross Cut Sled runners is hardwood and typically 2 are used. However, I saw a couple of plans that use aluminum as the runners and even one that suggests a single miter gauge bar as the only runner.

http://www.finewoodworking.com/item/...-crosscut-sled

I think I see advantages and disadvantages of all options, but I'd like to hear your opinions.

"2 hardwood runners is the way that it's always been done" will not automatically be considered an "advantage". ;-)